This invention relates to a plastic optical fiber continuous optical amplifier or continuous optical amplifier for a plastic optical fiber, with continuous wave pump light. Also, this invention relates to an excitation tube for continuously amplifying a signal light by continuous wave light irradiation from a side of the plastic optical fiber.
Recently, a graded index type plastic optical fiber has been developed and an information transmission efficiency has been improved significantly. Since the plastic optical fiber with a high bandwidth/low loss has been developed, the information transmission with an optical fiber communication network is going to be used practically.
The plastic optical fiber (hereinafter referred to as "POF") amplifier amplifies the amplitude of a propagating signal light with an optical excitation. A POF amplifier which provides a pulse amplification is disclosed in Japanese Patent Publication (KOKAI) No. 5-275789. In this amplifier signal light and pump light are put in from an end face of POF doped with organic dye. This excitation is referred to as a coaxial direction excitation.
Also, in an amplifier disclosed in Japanese Patent Publication (KOKAI) No. 9-8388, there is mentioned a transverse excitation, wherein a pump light is emitted from an exciting light source; an optical amplification fiber is irradiated from the side face by the pump light to thereby form an inversion distribution (population inversion); and the light to be amplified is amplified by propagating in an axial direction of a core where the population distribution is formed, to thereby output the amplified signal light from an optical connector.
However, in the conventional POF amplifier as disclosed in Japanese Patent Publication No. 5-275789, since a pulse light is used as the pump light, the pump light is required to be synchronized with the signal light. Especially, in case a super high speed optical transmission is carried out in a field of communication, the synchronization is very difficult, so that a device for synchronizing becomes extremely complicated, and also, a laser generating device with a large output is necessary as an pump light generating device.
Further, although it is preferable to use the POF in view of selection of an induced emission material in order to carry out the optical amplification in a wide wavelength region, when high power pulsed laser as a pump source is irradiated for longer period, there is a problem to be solved such that a resin of the POF is dissolved.
Also, in the example disclosed in Japanese Patent Publication No. 9-8388, although pump light is irradiated from the side face of fiber, it remains in a concept of irradiating the pump light from only the side face, and no specific transverse excitation method nor exciting device that can be practiced actually is disclosed.
On the other hand, it is indispensable to provide an optical fiber network to each home by arrival of a multi-media society. As an equipment for connecting each home with a main network formed of glass series optical fibers, a POF with a large diameter, a connecting work of which is easy at a low cost, is likely to be introduced.
However, as to the glass series optical fiber, a continuous wave (hereinafter referred to as "CW") operating optical amplifier has already been used practically, but there is no CW operating optical amplifier for the POF. An optical amplifier having a simple structure at a cheaper cost, which can be introduced into a branch point or a home, has been demanded.
The present invention has been made in view of the above, and an object of the present invention is to provide a continuous optical amplifier for a POF for continuously amplifying a signal light.
Another object of the invention is to provide an excitation tube for an optical amplification by exciting a doped fluorescence or phosphorescence material, which will induce the induced emission (hereinafter this kind of material is referred to as "induced emission material) in POF amplifier.
A further object of the invention is to provide an excitation tube as stated above, which can be produced with dimensions to be used practically.
Further objects and advantages of the invention will be apparent from the following description of the invention.